In recent years, reduction in the size and weight of the portable electronic apparatus is remarkable. With this tendency, there has been a growing demand for the reduction of the size and weight of the secondary battery as a power supply. In order to meet this requirement, various secondary batteries have been developed. Nowadays, a lithium ion battery comprising a positive electrode made of lamellar lithium-cobalt composite oxide as a positive active material has a high working voltage and a high energy density and thus is useful for the foregoing purpose and has been widely used. Lithium-cobalt composite oxide occurs scarcely and thus is expensive. Therefore, as a positive active material substituting for lithium-cobalt composite oxide, there has been proposed lithium-manganese composite oxide or lithium-nickel composite oxide.
However, lithium-manganese composite oxide is disadvantageous in that it has a low theoretical capacity density and shows a great capacity drop with charge and discharge cycles. Further, lithium-nickel composite oxide has the highest theoretical capacity density but is disadvantageous in that it exhibits deteriorated cycle life performance and thermal stability. A lithium-nickel composite oxide comprising lithium in a molar ratio which is not completely stoichiometrical can easily have an incomplete hexagonal structure having the Ni element incorporated in Li layer sites and thus easily cause deterioration of the cycle life performance.
In the case of large-sized battery, when a large current flows due to shortcircuiting, misuse, etc., the battery temperature suddenly rises, making it likely that a combustible liquid electrolyte or its decomposition gas can flow out and further can be ignited. In particular, when a lithium-nickel composite oxide is used as a positive active material, it releases oxygen at high temperatures while being charged because of the deteriorated thermal stability. Thus, there is a fear of causing a sudden reaction of the electrode with the liquid electrolyte that leads to thermal runaway and ignition/rupture of the battery.
The method for evaluating the safety of these batteries include the nail penetrating test defined in “Guideline for Criterion on Evaluation of Safety of Lithium Secondary Battery (SBA G101)” published by Nihon Chikudenchi Kogyokai (Japan Society of Storage Battery Industry). In accordance with this method, a nail having a diameter of from 2.5 mm to 5 mm is allowed to pierce through a fully charged battery at a substantially central portion at room temperature perpendicularly to the plane of the electrode. The battery is then allowed to stand for 6 hours or longer. This test is designed on the supposition that the battery can encounter misuse such as accidental penetration of a nail or the like during the packaging (e.g., in a wood box). When a nail pierces through the battery, the positive electrode and the negative electrode come in direct contact with each other in the battery to cause internal shortcircuiting. Accordingly, this method is also used to evaluate the possibility of ignition or rupture due to heat generation by sudden reaction in the battery.
In the foregoing nail penetrating test, it has been confirmed that the existing lithium secondary battery can undergo rupture/ignition. Therefore, it has been desired to develop a technique for enhancing the thermal stability of the battery without impairing the high performance thereof.
In order to provide the battery with high resistance to internal shortcircuiting or high safety, various mechanisms have heretofore been proposed. For example, a technique has been proposed which is designed to fuse a separator made of a porous membrane to close its pores and hence cause shutdown. Another technique involves the attachment of a PTC element which raises its resistivity with the temperature rise to the exterior of the battery. In this arrangement, when any abnormality occurs, flowing current gradually decreases.
However, it is essentially necessary that the safety of the secondary battery should be enhanced to prevent the occurrence of dangerous conditions even upon unforeseen accident. At present, it is difficult to say that the safety of the battery can be sufficiently established. In particular, a large-sized secondary battery having a capacity of 3 Ah or higher has an increased chemical energy stored in the battery. Thus, it is more important for this secondary battery to have a sufficient safety.
Under these circumstances, an object of the present invention is to provide a lithium-nickel composite oxide having a high capacity density and improved charge and discharge cycle life performance and thermal stability and provide a non-aqueous electrolyte secondary battery having a higher safety comprising such a lithium-nickel composite oxide as a positive active material.